Method of preparing ferrous metal for heat-treatment



Patented July 11, 1950 METHOD OF PREPARING FERROUS METAL FOR HEAT-TREATMENT Hugh G. Webster and John H. Shoemaker, De-

troit, Mich., assignors to Kolene Corporation,

Detroit, Mich.

No Drawing. Application February 5, 1945, Serial N 0. 576,375

3 Claims. 1

This application relates to the art of treating metals, and discloses a metal treating process. In describing the process of this application, We will relate it to an iron work piece, partly coated with scale, and partly machined.

Specifically, the process hereof prepares an iron work piece for a subsequent heat treating or forging operation in such a manner that objectionable decarburization and scaling of the work piece during such subsequent operation will not arise.

The first step is the immersion of the work piece in a molten salt bath consisting of about 35% of sodium hydroxide, about sodium chloride, and about 1% sodium aluminate. The work pieces, for immersion, may be deposited in a perforated metal basket. An electric current is then applied to that metal basket of such a nature that the work pieces, or the basket, forms a cathode, with the bath receptacle or any other element forming the anode. The combined activity of the bath and the electric current reduces the scale, which is an iron oxide, by the action of nascent reducing agents which are released and which transform that scale into iron.

Thereu-pon the current is reversed so that the work pieces of the basket containing them is made the anode, and the receptacle or some other element is made the cathode. Thereupon nascent oxidation agents are released at the anode and oxidizin takes place. pure iron oxide. Sulphur in the surface is transformed into sodium sulfate which is soluble and dissolves off the work piece and into the bath. The silicon becomes silicon fluoride, most of The iron scale becomes Thereafter the current is reversed again so that the work pieces or the basket containing them is a cathode and reduction takes place. The iron oxide is reduced to pure ferrite.

The foregoing are the preliminary steps of the process.

Thereafter, the workpiece, now having a pure ferrite surface is nickel plated by any suitable nickel plating process and this completes the preparation of the work piece for the subsequent operation of forging or heat treating.

A work piece thus prepared will not be subject to decarburization or scaling during the subsequent forging or heattreating operation.

Sodium aluminate causes the hydrogen and the sodium at the cathode to combine with other members to form soluble reducing agents; at the anode it causes the oxygen to combine with other members to form soluble oxidation agents.

It is possible to repeat the preliminary steps of the process (prior to nickel plating), as much as needed, without taking the Work piece out of the bath, and merely by continued applications of current, reversing the current from time to time so that alternate oxidation and reduction takes place. This may be prolonged to any extent needed in order to continue the treatment which escapes as fumes, the rest dissolving in the bath as sodium silicate. Carbon and grease are transformed first into carbon dioxide. Part of this carbon dioxide escapes as a gas, and part becomes a carbonate which dissolves into the bath. This sodium carbonate which dissolves into the bath becomes an electrolyte in the bath. This will be released electrolytically at the anode or tank when the current is reversed and the tank becomes the anode. Other ingredients such as phosphorous, manganese, aluminum, molybdenum, zinc, etc. present in the surface are removed in a manner similar to the removal of the sulphur. This leaves the surface a thick layer of iron oxide completely free of the foregoing ingredients, which, from the point of view of coating, at least, are deleterious and therefore may be regarded as impurities, though, of course, from the point of view of metal characteristics, are desirable.

of the metal and to attain a higher and higher degree of purity of the ferrite surface until the desired maximum is attained.

If the work piece is covered with scale, partly or wholly, the current is applied to the work piece as a cathode for reduction; however, if there is no scale, the cathode reduction step may be omitted and the first application of current may be the anode oxidation current.

If the casting has sand on its surface the sand may be removed first by the anode oxidation step. About 4% sodium fluoride is included in the bath; this will affect the sand to form silicon fluoride which is a gas and escapes, or to form sodium silicate. Thereafter the cathode reduction step may be applied to eliminate scale if scale is present.

If the work piece contains tungsten or other ingredients, which are insoluble in the bath, even with the electric current applied, then we first use the preliminary steps of the process hereof (prior to nickel plating) and thereafter immerse the work piece in a suitable acid to remove the tungsten or similar ingredients and then we repeat these steps to the extent necessary to get the desired results.

The bath itself, consists of the following ingredients:

1. Eighty-five percent by weight of sodium hydroxide; this operates to combine with the oxides being formed to form sodium salts which dissolve in the bath.

2. Ten percent by Weight of sodium chloride which functions as a catalyst to activate the oxygen to greater activity.

3. Optional, if sand is to be removed.) Four percent by weight of sodium fluoride which takes care of the silicon and the sand by forming silicon fluoride and sodium silicate.

4. One percent by weight of sodium aluminate which acts as a catalyst.

The bath herein is a fused molten salt bath and not a water bath. The temperature of the bath may be between 600 and 1000 degrees Fahrenheit, but optimum results have been obtained at 850-900 degrees Fahrenheit.

The work need not necessarily be connected to the circuit to form an anode or a cathode. All that is required is that the work be near enough to an anode or a. cathode, so that the nascent reducin agents and the nascent oxidation agents released at the anode or the cathode maintains that part of the bath, near the work, in an oxidizing or reducing condition, as needed. This makes it possible to place a large number of work pieces in a metal basket which serves as the anode or cathode and makest'he preliminary processing of such small work pieces far simpler and far more economical than would be the case if each work piece individually were required to be electrically connected by itself to form an anode or cathode.

, The electric current operates to break down the bath and not the work piece, into either a reducing or an oxidizing bath in the vicinity of the work piece. When the work piece or its holder is a cathode, nascent reducing agents are given ofi at the work piece for reducing and when it is an anode, nascent oxidation agents are given off for oxidizing. I

The electrical current is slow alternating or successively reversed direct, of 4 to 6 volts, with an intensity of 50 amperes per square foot. I have found that five minutes has proven satisfactory for oxidizing and ten to thirty minutes for reduction.

The bath is activated electrically but the Work piece is not dissolved, instead its surface is alternately oxidized and reduced.

The bath is alkaline, and not acid. The process hereof transforms scale, first by reducing it into. iron and then by oxidizing the iron and again reducing it to form pure ferrite. No pickle smudge is formed nor is the. base metal at the surface dissolved, U i

,No hydrogen absorption or embrittlement takes place because there is no free hydrogento be absorbed.

Further, no etching takes place, because there is no acid or metal attacking reagent formed. The metal is not eaten away nor is there any loss in the predominant base metal. The only thing removed from the surface are the ingredients other than the pure ferrite and this is left on ,the surface. I

A novel step in the process is the step of electrically activating the bath to be alternatingly reducing or oxidizing in the vicinity of the work, without dissolving the surface of the work piece. This operates to oxidize or reducethesurface of the work piece, alternately rather than dissolving the surface. This results in the surface being, of

an extremely high degree of purity of ferrite.

4 The ferrite layer formed by the process hereof can be made considerably thicker than the ferrite layer formed by any previously known process.

It will be observed that if graphite is not removed, the surface will be composed of graphite and ferrite, and since these are of different electrochemical values, an intercell action may result. However, if graphite is removed, and ferrite only is left on the surface, no cell exists and no inter-cell action takes place.

The nickel plating is applied to prevent decarburization and the formation of objectionable scale during the heat treating or forging operation. Instead of nickel, copper may be applied as the plating, for the same purpose. Other platings may be known but for the present nickel and copper are well known for this purpose.

It will be observed that the process hereof does in fact decarburize or eliminate carbon from the thin layer of ferrite at the surface of the Work piece, but that isof the express purpose of forming a good surface for bonding-namely a carbon free surface. Therefore, when we say that the process hereof prepares a work piece in such a manner that objectionable decarburization and scaling of the work piece during subsequent heat treating and forging operations, will not occur, we really mean that the body of the work piece is not decarburized or objectionably scaled during the heat treating or forging operations, even though the pure ferrite layer, useful for bonding, is a carbon free surface. The objective of the invention is not impaired by the fact that a minute bonding or surface layer of the work piece islactually decarburized or made free of carbon.

Now having described the process hereof, reference should be had to the claims which. follow:

We claim:

1. In preparing metal stock forheat treatment preliminary to forging, the method comprising immersing a scale and oxide coated ferrous metal workpiece in a molten salt bath consisting essentially of a fused mixture of sodium hydroxide, sodium chloride and sodium aluminate with the sodium hydroxide predominating in proportions of several times that of the other components and the sodium aluminate being added in a quantity of about one part by weight of the mixture, directing successively reversed direct currents into the bath until the surface of the Work piece is converted to a superficial layer of substantially pure ferrite and then plating the work piece with a metal selected from the group consisting of nickel and copper.

2. In preparing metal stock for heat treatment preliminary to forging, the method comprising immersing a scale and oxide coated ferrous metal workpiece in a molten salt bath consisting essentially of a fused mixture of sodium hydroxide, sodium chloride and sodium aluminate in approximate proportions by weight o'f85 parts of sodium hydroxide, 10 parts of sodium chloride and 1 part of sodium aluminate, directing successively reversed direct currents into the bath until the surface of the work piece is converted to asuperficial layer of substantially pure ferrite and then plating the work piece with a metal selected from the group consisting of nickel and copper.

8. In preparing metal stock for heat treatment preliminary to forging, the method comprising immersing a scale and oxide coated ferrous metal workpiece in a molten salt bath consisting essentially of a fused mixture of sodium hydroxide, sodium chloride, sodium fluoride and sodium aluminate in approximate proportions by weight of 85 parts of sodium hydroxide, 10 parts of sodium chloride, 4 parts of sodium fluoride and 1 part of sodium aluminate, directing successively reversed direct currents into the bath until the surface of the work piece is converted to a superficial layer of substantially pure ferrite and then plating the work piece with a metal selected from the group consisting of nickel and copper.

HUGH G. WEBSTER.

JOHN H. SHOEMAKER.

REFERENCES CITED The following references are of record in the file of this patent:

OTHER REFERENCES Metals Handbook, 1936 edition, pp. 785-787. Iron and Steel, June 1944, pp. 501-505. 

1. IN PREPARING METAL STOCK FOR HEAT TREATMENT PRELIMINARY TO FORGING, THE METHOD COMPRISING IMMERSING A SCALE AND OXIDE COATED FERROUS METAL WORKPIECE IN A MOLTEN SALT BATH CONSISTING ESSENTIALLY OF A FUSED MIXTURE OF SODIUM HYDROXIDE, SODIUM CHLORIDE AND SODIUM ALUMINATE WITH THE SODIUM HYDROXIDE PREDOMINATING IN PROPORTIONS OF SEVERAL TIMES THAT OF THE OTHER COMPONENTS AND THE SODIUM ALUMINATE BEING ADDED IN A QUANTITY OF ABOUT ONE PART BY WEIGHT OF THE MIXTURE, DIRECTING SUCCESSIVELY REVERSED DIRECT CURRENTS INTO THE BATH UNTIL THE SURFACE OF THE WORK PIECE IS CONVERTED TO A SUPERFICIAL LAYER OF SUBSTANTIALLY PURE FERRITE AND THEN PLATING THE WORK PIECE WITH A METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL AND COPPER. 